In the applications considered, the power source of the device may comprise a rechargeable battery which can be recharged using a charger providing charging voltage from the power grid. The supply voltage available ranges from about 4.8 V when the voltage is provided by the charger, to about 2.3 V which corresponds to the minimum charging voltage of the battery (below which proper operation can no longer be assured). The voltage supplied by the fully charged battery is about 3.6 V.
In audio applications, the output current that the regulator must provide is typically between 10 and 20 mA, depending on the listening volume and the intensity of the sound to be reproduced. This range of values corresponds to listening at about two-thirds of the maximum power. However, to allow listening at a higher volume and/or to support peak power draw, the regulator must be able to deliver an output current on the order of 120 to 130 mA.
The voltage supplied by the battery may be low when the battery charge is low, for example after prolonged use. Battery voltage can also be affected by temperature, as the supply voltage it delivers is lower when the temperature is lower.
The resistivity of the MOS (Metal Oxyde Semiconductor) transistors used in the regulator is also highly dependent on temperature.
Depending on the specifications to be met in the type of applications considered, the voltage regulator of the device must be able to deliver the reference voltage even if the load consumes a high amount of current, for example 120 to 130 mA, and even when the supply voltage delivered by the battery falls to the minimum value of about 2.3 V.
In order to ensure compliance with these specifications, the voltage regulator can comprise a charge pump circuit in which the switches are typically realized with power MOS transistors. To maintain a stable output voltage in spite of a high output current and a low supply voltage, the resistivity of the MOS transistors must be as low as possible. This is why the MOS transistors are large in size. Typically, their gate width is about 0.5 μm, and their gate length is about 56 mm for a PMOS transistor or about 28 mm for an NMOS transistor. Their gate is typically folded with multiple fingers on the silicon substrate. For brevity, such transistors are called “large transistors” in the following description.
However, the parasitic gate-source capacitance of such transistors is high, and the charge pump circuit therefore has significant switching loss. This affects the power consumption of the circuit. In fact, the power dissipated because of this switching loss, denoted below as P, is expressed by the following relation:P=Cgs×F×V2  (1)where                Cgs indicates the gate-source capacitance;        F indicates the switching frequency; and,        V indicates the supply voltage of the MOS transistor.        
To reduce switching loss, one can use the “pulse skipping” technique borrowed from the domain of DC-DC converters. This technique consists of reducing the switching frequency by skipping pulses of the control signal for the charge pump switches when an error signal, corresponding to the difference between the output voltage from the regulator and a reference voltage, exceeds a given threshold. This technique, however, only applies for very low values of current absorbed by the load.
FIG. 1 shows a graph of the operating region in which pulses can be skipped in this manner, inside the operating range 11 of the voltage regulator defined by the value of the supply voltage Vdd and the value of the output current Iout. The current Iout is the current delivered by the voltage regulator, meaning the current absorbed by the load. In general, the voltage regulator operates for values of Vdd of between 2.3 V and 4.8 V and values of Iout which can reach several hundred milliamps. Within this range 11, one can see that pulse skipping can be performed for values of the current Iout that are on the order of several milliamps only, with these values growing lower as the value of Vdd falls. Typically, pulse skipping can only be considered for a value of Iout of about 1 mA when Vdd is equal to 2.3 V, or for a value of Iout of about 3 mA when Vdd is equal to 4.8 V.